Techniques have long been known for depositing substances, such as layers of semiconductor material, using a plasma that is formed into a jet. For example, U.S. Pat. Nos. 4,471,003 and 4,487,162 disclose arc jet plasma deposition equipments which utilize a plasma for deposition of semiconductors and other materials. Ions and electrons are obtained by injecting an appropriate compound, such as a silicon compound, into an arc region, and a jet (or beam) is formed by utilizing magnetic fields to control the plasma. Recently, equipment of this type has been used to deposit synthetic diamond. Superior physical and chemical properties make diamond desirable for many mechanical, thermal, optical and electronic applications, and the ability to deposit synthetic diamond by plasma jet deposition holds great promise, particularly if plasma jet techniques can be improved for this and other purposes. A plasma of a hydrocarbon and hydrogen can be obtained using electrical arcing, and the resultant plasma directed toward a substrate, so that polycrystalline diamond film is deposited on the substrate. Reference can be made, for example, to U.S. Pat. No. 5,204,144 for description of an example of a type of plasma jet deposition that can be utilized to deposit synthetic diamond on a substrate.
In various commercial applications it is desirable to have relatively large size diamond films. In plasma jet deposition techniques there are various factors which limit the practical size of the deposition area that is active on a substrate at a particular moment. For example, when an arc is employed to generate the heated gas mixture in an arc jet plasma deposition system, the diameter of the beam can be limited by a number of factors. Since the cross-section of the plasma beam is generally limited in practical applications, the area on which it is desired to deposit a diamond film may be larger than the deposition beam. This means that it may be desirable to move the beam and the target substrate with respect to each other during the deposition process. This has been achieved by spinning the substrate during deposition, which helps to promote temperature and diamond quality uniformity over the substrate, as well as to attain larger area substrate coverage (see e.g. the referenced U.S. Pat. No. 5,204,144).
In plasma jet deposition of the type described, it is generally necessary to cool the substrate (or mandrel) upon which the diamond is being deposited, to prevent the hot plasma from overheating the deposition surface, and to provide an optimum deposition temperature for the particular product characteristics desired. A coolant can be circulated in the mandrel to provide cooling. In a rotating mandrel type of deposition equipment, as described in U.S. Pat. No. 5,204,144, cooling fluid can be circulated through a rotating union. In copending U.S. patent application Ser. No. 08/175,586, now U.S. Pat. No. 5,486,380, assigned to an assignee of the present Application, there is disclosed a cooling arrangement for a rotating mandrel assembly in which the mandrel has extending radiator fins that interleave with stationary receptor fins which are water cooled. In an embodiment of the copending Application, heat transfer between the radiator and receptor fins can be modified by changing the gas between radiator and receptor fins to change the thermal conductivity therebetween.
In U.S. Pat. No. 5,204,125 there is disclosed CVD plasma torch diamond deposition on rotating substrates in an environment of hydrogen and inert gas. Cooling plates, which do not rotate, are provided, and each cooling plate can be brought closer or further from a substrate to maintain the substrate at a set temperature.
In plasma CVD deposition of diamond film on the surface of a substrate disc, it is typical to employ a spacer between the underside of the substrate and a cooling block (or cooled mandrel) that is used to cool the substrate which receives a high heat flux from the hot plasma beam. The spacer, which may be, for example, a graphite disc, facilitates the temperature transition between the bottom of the substrate (which may be, for example, at a temperature in the range 500 to 1100 degrees C) and the top surface of the cooling block (which may be, for example, at a temperature in the range 10 to 500 degrees C). Typically, the substrate, spacer, and cooling block are held together by bolting or clamping, and the composite thermal conductivity through these components is quite sensitive to the relative surface contours of the components and to the pressure with which the surfaces are pressed together. Non-uniformity of contact and/or pressure can lead to substantial temperature differences at the deposition surface which, in turn, can degrade the uniformity and quality of the diamond film being deposited. Also, if contact and/or pressure changes when a new substrate is put in place, the substrate temperature will be different during deposition. The resultant lack of repeatable deposition conditions can also tend to degrade the overall quality of the diamond film being produced.
One of the disadvantages of requiring a relatively uniform pressure on the substrate/spacer/mandrel sandwich is that each time a new substrate is mounted, it is necessary to adjust the bolting or clamping to obtain the desired pressure uniformity. The deposition chamber operates at low pressure, and it takes considerable time for the vacuum pumping system to reestablish the appropriate pressure before deposition can be initiated on the newly mounted substrate. It would be advantageous to be able to change substrates within the deposition chamber without breaking vacuum (such as by using a robot arm), but the need for securing the substrate and the spacer to the cooling block with precision renders the task more difficult.
It is among the objects of the present invention to provide an improved apparatus and technique for temperature control of a substrate in a deposition system, such as for deposition of diamond film by CVD plasma jet.